Tempering

Tempering by the Color Method. -- Hardened steel can be tempered or made softer and less brittle by re-heating it to a certain temperature (depending on the nature of the steel and its intended use), and then cooling. When steel is tempered by the color method, the temper is gaged by the colors formed on the surface as the heat increases. First the surface is brightened to reveal the color changes, and then the steel is heated either by placing it upon a piece of red-hot metal, a gas-heated plate or in any other available way. As the temper increases, various colors appear on the brightened surface. First there is a faint yellow which blends into straw, then light brown, dark brown, purple, blue and dark blue, with various intermediate shades. The temperatures corresponding to the different colors and shades are given in the table on temperatures and colors for tempering. Turning and planing tools, chisels, etc., are commonly tempered by first heating the cutting end to a cherry-red, and then quenching the part to be hardened. When the tool is removed from the bath, the heat remaining in the unquenched part raises the temperature of the cooled cutting end until the desired color (which will show on a brightened surface) is obtained, after which the entire tool is quenched. The foregoing methods are convenient, especially when only a few tools are to be treated, but the color method of gaging temperatures is not dependable, as the color is affected, to some extent, by the composition of the metal. The modern method of tempering, especially in quantity, is to heat the hardened parts to the required temperature in a bath of molten lead, heated oil, or other liquids; the parts are then removed from the bath and quenched. The bath method makes it possible to heat the work uniformly, and to a given temperature with close limits.

High Temperatures judged by Color, and Colors for Tempering

Degrees Centi- grade

Degrees Fahren- heit

High Temperatures judged by Color

Degrees Centi- grade

Degrees Fahren- heit

Colors for Tempering

400

752

Red heat, visible in the dark

221.1

430

Very pale yellow

474

885

Red heat, visible in the twilight

226.7

440

Light yellow

525

975

Red heat, visible in the daylight

232.2

450

Pale straw-yellow

581

1077

Red heat, visible in the sunlight

237.8

460

Straw-yellow

700

1292

Dark red

243.3

470

Deep straw-yellow

800

1472

Dull cherry-red

248.9

480

Dark yellow

900

1652

Cherry-red

254.4

490

Yellow-brown

1000

1832

Bright cherry-red

260.0

500

Brown-yellow

1100

2012

Orange-red

265.6

510

Spotted red-brown

1200

2192

Orange-yellow

271.1

520

Brown-purple

1300

2372

Yellow-white

276.7

530

Light purple

1400

2552

White welding heat

282.2

540

Full purple

1500

2732

Brilliant white

287.8

550

Dark purple

1600

2912

Dazzling white (bluish-white)

293.3

560

Full blue

298.9

570

Dark blue

Tempering in Oil. -- Oil baths are extensively used for tempering tools (especially in quantity), the work being immersed in oil heated to the required temperature, which is indicated by a thermometer. It is important that the oil have a uniform temperature throughout and that the work be immersed long enough to acquire this temperature. Cold steel should not be plunged into a bath heated for tempering, owing to the danger of cracking it. The steel should either be pre-heated to about 300 degrees F., before placing it in the bath, or the latter should be at a comparatively low temperature before immersing the steel, and then be heated to the required degree. A temperature of from 650 degrees to 700 degrees F. can be obtained with heavy tempering oils; for higher temperatures, a lead bath is generally used. A tempering oil which has given satisfactory results in practice has the following characteristics: Composition, mineral oil, 94 per cent; saponifiable oil, 6 per cent; specific gravity 0.920; flash point, 550 degrees F.; fire test, 625 degrees F. The foregoing figures apply to new oil. When the oil has been used long enough to be rendered practically useless, an analysis shows the following changes: Composition, mineral oil, 30 per cent; saponifiable oil, 70 per cent; specific gravity, 0.950; flash point, 475 degrees F.; fire test, 550 degrees F. The great difference in the composition of new and old oil is due to the loss of mineral oil, resulting from the high heats to which tempering oil is frequently or constantly subjected; hence, the durability of the tempering bath can be increased by occasionally adding new mineral oil.

Flash Point and Fire Test. -- The distinction between the "flash point" and the "fire test" of an oil is as follows: The flash point is the temperature at which the amount of vapor given off is sufficient to form an inflammable or explosive mixture with the air over the surface of the oil, so that the gaseous mixture ignites and burns with a momentary flash when a flame is applied. As the temperature of the oil rises, more vapor is given off, and then the production of vapor is rapid enough to maintain a continuous flame, the oil takes fire and burns. The temperature at which this occurs is called the fire test, firing point or burning point of the oil.

Tempering in a Lead Bath. -- The lead bath is commonly used for heating steel preparatory to tempering, as well as for hardening. The bath is first heated to the temperature at which the steel should be tempered; the pre-heated work is then placed in the bath long enough to acquire this temperature, after which it is removed and cooled. As the melting temperature of pure lead is 618 degrees F., tin is commonly added to it to lower the temperature sufficiently for tempering. Reductions in temperature can be obtained by varying the proportions of lead and tin, as shown by the table, "Temperatures of Lead Bath Alloys".

Temperatures of Lead Bath Alloys

Parts Lead

Parts Tin

Melting Temp., Deg. F.

Parts Lead

Parts Tin

Melting Temp., Deg. F.

Parts Lead

Parts Tin

Melting Temp., Deg. F.

200

8

560

39

8

510

19

8

460

100

8

550

33

8

500

17

8

450

75

8

540

28

8

490

16

8

440

60

8

530

24

8

480

15

8

430

48

8

520

21

8

470

14

8

420

To Prevent Lead from Sticking to Steel. -- To prevent hot lead from sticking to parts heated in it, mix common whiting with wood alcohol, and paint the part that is to be heated. Water can be used instead of alcohol, but in that case the paint must be thoroughly dry, as otherwise the moisture will cause the lead to "fly". Another method is to make a thick paste according to the following formula: Pulverized charred leather, 1 pound; fine wheat flour, 1-1/2 pound; fine table salt, 2 pounds. Coat the tool with this paste and heat slowly until dry, then proceed to harden. Still another method is to heat the work to a blue color, or about 600 degrees F., and then dip it in a strong solution of salt water, prior to heating in the lead bath. The lead is sometimes removed from parts having fine projections or teeth, by using a stiff brush just before immersing in the cooling bath. This is necessary to prevent the formation of soft spots.

Pots for Lead Baths. -- Melting pots for molten lead baths, etc., should, preferably, be made from seamless drawn steel rather than from cast iron. Experience has shown that the seamless pots will sometimes withstand six months' continuous service, whereas cast iron pots will last, on average, only a few days, under like conditions. Cast steel melting pots, if properly made, are as durable as those made of seamless drawn steel.

(Fig. 9) Arrangement used for Sand Tempering.

Tempering in Sand. -- The sand bath is used for tempering certain classes of work. One method is to deposit the sand on an iron plate which is heated by suitable means as indicated in the accompanying illustration, Fig. 9. With this method of tempering, tools such as boiler punches, etc., can be given a varying temper by placing them endwise in the sand. As the temperature of the sand bath is higher toward the bottom, a tool can be so placed that the color of the lower end will be a deep dark blue when the middle portion is a very dark straw, and the working end or top a light straw color, the hardness gradually increasing from the bottom up. Tools to be heated by this method must be polished, as the temper is judged by the color. for tempering parts in quantity, sand tempering machines have been developed. One well-known design has a horizontal revolving cylinder containing rows of perforated pockets which become filled with sand in steady streams upon the work. The drum revolves at different rates of speed for different classes or work, usually making from 3 to 10 revolutions per minute. The heat is supplied by a gas burner. The machine is equipped with a thermometer, which does not indicate the actual temperature of the sand, but a somewhat lower temperature than would be required for the same tempering color, under other conditions. The thermometer reading, therefore, is relative and not a precise indication of the tempering temperature.

(Fig. 10) Tempering by utilizing a Heated Inclined Plate on which the Objects roll down to the Cooling Bath.

A plate arranged as shown in Fig. 10 will be found very convenient when drawing small, round pieces. The pieces are rolled on the inclined plate which is heated as indicated. The length of time the work is in contact with the plate can be regulated by adjusting the amount of the incline, as well as the location of the "stop". This arrangement can also be used for such work as punches, etc., in which case the plate, of course, should stand level and not in an inclined position.

Tempering Temperatures for Various Tools

Degrees F.

Class of Tool

495 to 500

Taps 1/2 inch or over, for use on automatic screw machines

490 to 495

Taps 1/2 inch or over, for use on screw machines where they pass through the work

Snaps for pneumatic hammers -- harden full length, temper to 460 degrees, then bring point to 520 degrees.

Tempering Furnaces. -- In tempering furnaces the only really important consideration is to insure that the furnace is so built as to heat the bath uniformly throughout. It is doubtful if there can be found a tempering furnace on the market that will fill this requirement entirely, although many give good results in general. It is never safe, however, to let any tools being tempered rest against the bottom or sides of the tank, as no matter how scientifically the furnace may be built these parts are, in most cases, hotter than the fluid itself. It is, of course, just as important not to let the thermometer rest against any of these parts in order to insure correct readings. After the pieces tempered are taken out of the oil bath, they should immediately be dipped in a tank of caustic soda (not registering over 8 or 9), and after that in a tank of hot water. This will remove all oil which might adhere to the tools.

Fig. 7 shows an ordinary type of tempering furnace. In this the flame does not strike the walls of the tank directly. The tools to be tempered are laid in a basket which is immersed in the oil. In Fig. 8 is shown a tempering furnace in which means are provided for preventing the tools to be tempered from coming in contact with the walls or bottom of the furnace proper. The basket holding the tools is immersed in the inner perforated oil tank. The same arrangement can, of course, be applied to the furnace shown in Fig. 7.

In tempering, the best method is to immerse the pieces to be tempered in the oil before starting to heat the latter. They are then heated with the oil.

Tempering High-Speed Steel. -- Heavy high-speed tools having well-supported cutting edges (such as large planing or turning tools) are often used after hardening and grinding, without tempering. Tools that are comparatively weak should be toughened by tempering to suit the particular service required. The steel is generally heated in a bath of lead, oil, or salts. The tempering temperatures recommended by high-speed steel manufacturers usually vary from 400 degrees to 1000 degrees F., so that definite information should be obtained from the maker of the particular steel to be used. One well-known manufacturer recommends re-heating hardened lathe tools to 1000 degrees F., and tools such as milling cutters, taps, dies, etc., to 500 degrees or 650 degrees F. According to another manufacturer, it is desirable to temper most high-speed steel tools in order to make them more resistant to shocks, the drawing temperatures varying from 600 degrees to 1100 degrees F. Still another steel maker advises tempering lathe and similar tools to 950 degrees F. Lower temperatures varying from 400 degrees to 500 degrees F. are sometimes recommended for tools such as cutters, dies, reamers, etc.